1. Field of the Invention
This invention relates to an optical amplifier. Optical amplifiers are devices useful to optically (i.e., without conversion to electrical or other signal types) amplify light signals being propagated through a fiber optics line.
2. Description of Prior Art
In particular, so-called "on-fiber" amplifiers are known wherein the amplifier element comprises a section of an active light guide as obtained by a doping process using special substances (typically rare earths); in going through that light guide section, the optical signal is amplified by pumping light energy from laser sources.
Occasionally, the amplification is split between two amplification stages, that is, two discrete sections of the active light guide. Two-stage optical amplifiers have been known, for instance, from U.S. Pat. No. 5,115,338 and European Patent Application EP 0509577.
The implementation of optical amplifiers, particularly two-stage ones, is far from being a straightforward process, it involving, in particular, a large number of light guide splices; also, the process is made the more difficult by the need to keep the amplifier space requirements low.
The active light guide sections tend to be fairly long--not infrequently several tens of meters long--and cannot be convoluted to an excessively small bending radius for fear of attenuating the signal of possibly damaging the light guide beyond repair. In addition, the amplifier is to also accommodate a number of optical, electronic, and electrical components.
Lastly, several interconnection light guide sections must be provided between the various components which require to be sufficiently long; indeed, light guide splicing problems are likely to be encountered (during the amplifier assembly process), whereby some light guide excess length must be conceded to allow for reiterate splicing of the light guide.
Further, the splicing operation requires that purposely designed equipment be operated close to the amplifier outer case, on the exterior thereof for space reasons; it is also on this account that a sufficient length of the light guide should be made available for use as a starter to said equipment during the assembly process.
In the field of devices attendant to fiber optics lines, joining devices are known--particularly intended for light guides including a large number of individual fibers--which address the problem of providing for a rational setting of the light guide allowance within a case. Devices of this kind are known, for instance, from U.S. Pat. No. 4,428,645; U.S. Pat. No. 4,679,896; U.S. Pat. No. 4,687,289 and U.S. Pat. No. 4,619,499.
It should not be overlooked, however, that the problems affecting the joining devices differ from those of optical amplifiers. In fact, joints include no other optical or electrical components besides the light guide, whereby the difficulties inherent to the mutual optical connection of several components, as are typical of an optical amplifier, are non-existent there. Further, the demand for compact size is more stringent on an optical amplifier, despite this being a much more complicated construction than a joint; indeed, whereas a joint is normally a stand-alone component somewhere along a light guide, an amplifier requires to be housed inside a cabinet or a cabin together with other components.
In a typical arrangement including a single amplification stage, an optical amplifier would comprise optical components (with this term encompassing all its components, whether purely optical or opto-electrical, which have at least one input or output port for optical signals), such as a section of an active light guide, a dichroic coupler, a pump laser source arranged to act on the active light guide section through the dichroic coupler, a directional coupler associated with a respective optical meter, and light guide sections interconnecting the various optical components, as well as electronic components (meaning all those components which involve purely electrical connections and no optical connections) electrically connected together for powering and controlling the optical components.
Where plural amplification stages are used, the number of the components would increase; as an example, in a typical two-stage arrangement, the optical amplifier would comprise two sections of an active light guide, two dichroic couplers, two pump laser sources acting on the active light guide sections through the dichroic couplers, two directional couplers associated with two respective optical meters, and light guide sections interconnecting the various components.